Hot top compositions and method of preparing same



United States Patent This invention relates to a combustible composition and more specifically to a method of preparing a combustible hot top composition. Still more specifically, it relates to rigid forms of a combustible hot top composition and to a method of preparing these forms.

In preparing ingots, hot metal is poured into molds which contain at the top portion what is generally known in the trade as a hot top. These hot tops usually are prepared from a mixture of refractory and combustible materials and are placed in the uppermost portion of the mold for purposes of insulating and supplying heat .to the top of the ingot. Normally, without the use of these hot tops, the upper portion of the ingot cools at a rate much faster than the bottom and thus forms what is known as piping or shrinking. To avoid this, it is presently the practice of using hot tops as a means of bolding molten metal over the piping area while at the same time supplying or preventing heat from escaping from the top of the mold. .In following this practice, however, it has become increasingly necessary to improve the body strength and likewise the effectiveness of these hot tops.

There is a current need for combustible compositions which can be shaped into rigid forms and fitted in the top of any shape of mold. Here they may supply heat or act as ,aninsulator by preventing heat in the molten metal from escaping. The composition of these hot tops should be sufliciently strong that rigid forms can be made without requiring the use of outside reinforcements, such as castings and the like. The ingredients of the composition should impart to the hot top enough strength so that it can be formed into any desired shape which can be adjusted easily in the top of the mold in such a manner as to avoid movement. A failure to obtain a closely fitted hot top around the upper portion of the mold will result in the hot metal leaking and coming into contact with the inside of the walls of the mold, thus causing cracks or the like in the steel.

The composition of this invention provides a means of preparing such a hot top. The composition consists essentially of ground wood, calcium magnesium carbonate, and a sodium silicated binder in specific relative proportions. The calcium magnesium carbonate decomposes to give off CD, which keeps the heat at the top of the ingot by acting as an insulator while the wood, being exothermic, keeps the hot top from absorbing heat. The composition is prepared in such a manner as to provide hot tops that have a high body strength. This improved strength is what enables the formation of many different sizes and shapes. Thus, any desired shape or form that will hold the molten metal adequately can be made with the present composition. Different shapes have different advantages; thus, it is possible to shape the hot top so that it will fit firmly in the top against the inside walls of the mold and, if desired, extend beyond the top without breaking. It is possible, for example, to form a combustible hot top around the upper part of the molds by using rigid forms that are held closely together by such means as a tongue and groove. This close fitting prevents any movement or leaking which otherwise would cause the molten metal to come in contact with the sides of the molds. Because of the strength of the composition, it also is possible to shape the hot tops so that the "ice inside walls slope inwardly as they extend upwardly beyond the top of the mold. By using this particular shape, for example, a minimum amount of metal is held in the hot top and at the same time the hot top portion of the ingot is positioned over the area where the piping or cavity is most likely to occur.

Accordingly, it is an object of this invention to provide a composition for preparing relatively thin hot tops having improved body strength.

It is another object of this invention to provide a method of preparing hot tops having improved body strength.

It is still another object of this invention to provide a composition for preparing relatively thin hot tops of different sizes and shapes.

It is a further object of this invention to provide a hot top composition which can be shaped to fit tightly in the tops of ingot molds.

It is a still further object of this invention to provide a composition for preparing hot tops which are exother mic and will insulate heat from escaping from the upper portion of the ingot mold.

These and other objects of the invention will become apparent from a further and more detailed description of the invention.

It has been found that improved hot t'ops can be prepared from a composition consisting essentially of calcium magensium carbonate, ground wood, and sodium silicate. More specifically, hot tops of impro 2d body strength can be prepared from a composition comprising essentially 50 to 60 parts by weight of calcium magnesium carbonate, 12 to 18 parts by weight of sodium silicate, and 10 to 15 parts by weight of wood dust. The calcium magnesium carbonate is available commercially as dolomite, which is essentially calcium magnesium carbonate containing in addition a very small amount of silicate, iron, and aluminum oxide. Approximately 50 to 60 parts by weight of calcium magnesium carbonate,

and preferably between 54 and 58 parts by weight of the carbonate, are used as the refractory material. The wood dust, which is usually a hard wood having a particle size ranging from 10 to 30 mesh, is the combustible material and is added to the composition in the range of about 10 to 15 parts by weight, and preferably from 12 to 14 parts by weight. The wood dust and calcium magnesium carbonate are held together with approximately 12 to 18 parts by weight of sodium silicate binder, and preferably 15 to 16 parts by weight of the silicate. This mixture is then heated to a temperature as high as 380 F. to obtain a cured material having a substantially im proved body strength.

In preparing the hot top compositions, there are certain criticalities with respect to proportions and methods of mixing the ingredients that are essential to obtaining a product which has this superior body strength. It is this increased strength which distinguishes the instant hot tops from the thicker ones proposed heretofore. It was necessary, for example, to prepare hot tops from thick and heavy compositions, or with outside supports in order to obtain a hot top that could withstand the pressures created by the metal poured into them. The body strength of the instant hot tops, however, enables them to be used without outside support and still be sufiiciently'thin to be formed into many different shapes. It may be advisable in some instances, however, to use metal bands as supports. This may be so when the hot top is especially thin as required for a particular situation. The increased strength is imparted to these hottops by critically adjusting the relative proportions of the essential ingredients.

In preparing these hot tops, the specific quantities of calcium magnesium carbonate, wood dust, and sodium silicate are mixed in a batch type mixer with two curved blades to obtain uniform distribution of the components. This mixture is then formed into the desired shape by using a form box or by hand and then is dried and cured in an oven at temperatures ranging up to about 380 F. In the initial mixing of the ingredients, however, it is important that all the calcium magnesium carbonate be mixed with only approximately one-fifth of the total amount of sodium silicate to be used. These two ingredients are mixed until the carbonate has been covered completely with the silicate. At this point the mixture can be left standing for any length of time without any adverse effect on the final product. Just prior to forming the mixture into the desired shape, all of the wood dust is introduced into the mixture and thoroughly mixed with the silicate and carbonate. After the addition of the wood dust, then the remaining portion, or four-fifths, of sodium silicate is introduced into mixture. Here, it is essential to note that only part of the silicate is added initially to the calcium magnesium carbonate to avoid a complete reaction, and that the remainder of the silicate is mixed in after the wood dust has been added.

In preparing these hot tops, it was discovered that certain limitations with respect to the relative proporitions and the method of their preparation are critical in order to obtain a product having the desired body strength. It was found, for example, that by adding all of the alkaline silicate to the carbonate in the presence of the ground wood, it would cause the mixture to harden at a rate too fast for most commercial manufacturing processes; that is, too fast to permit the material to be shaped. This is particularly undesirable since it does not allow sufiicient time to make the shapes and designs desired. On the other hand, by adding all of the silicate to the carbonate prior to the addition of the ground wood also causes a reaction. This reaction occurs between the silicate and the carbonate without permitting the ground wood to be dispersed in the composition. Once the reaction goes to completion, there is no opportunity for the ground wood to become intimately dispersed or bound to the mixture. Thus, under ordinary stress and strain these products would crumble or break. Consequently, it wasdiscovered that the calcium magnesium carbonate can be inactivated or partially reacted at room temperature by the addition of only a portion of the alkaline silicate, with the remainder being added to the mixture after the addition of the ground wood. The addition of about 20 percent, or one-fifth of the total amount of the alkaline silicate to the carbonate is believed to reduce the reactivity such that the ground wood and the remainder of the silicate can be added at any point up to the time when the mixture is ready to be shaped into the desired hot top. It is this initial neutralization, or partial reaction of the carbonate that allows the mixture to be shaped into a particular design before it begins to stiffen or harden.

It is essential in obtaining good hot tops that the mixture have a reserve of alkalinity up to the time that the mixture is to be heated or cured. This heating or curing causes the reaction of the ingredients to go to completion, which results in a strong but top. The sodium silicate binder, usually, is an aqueous solution containing about 50 percent by weight of sodium silicate. The silicates used in preparing these 50 percent aqueous solutions may vary from Na O-%SiO to Na:O-3.9Si0, and differ in alkalinity from a pH of 13.2 to 10.8, respectively. A typical solution comprises about 53 percent by weight water, 31 percent by weight SiO and 13.9 percent by weight Na O. After the remainder of the basic silicate, e.g. about 80 percent of the total, is added to the mixture of calcium magnesium carbonate, ground wood, and silicate, the composition is formed either by hand, form boxes, or by any other means, to the desired shape and then heated to a temperature up to about 380 F. Here again, it is essential that the heating or curing be completed in a sequence of steps, such that the temperature does not substantially exceed 380 F. It is during the heating cycle that the reaction of the silicate with the carbonate goes to completion giving increased body strength to the finished product. In most case about 16 percent of the total mixture is water, added to the mixture as an aqueous solution of sodium silicate. Most of this water is removed at temperatures ranging up to about 380 F. After the composition has been heated up to about 380 F. and the majority of the water has been removed, the temperature then is lowered to about 360 F and finally to 300 F., or below, to obtain a cured product.

Initially, the temperature of the air in the drying oven can be as high as 380 F. because of the presence of water vapors. It is the water that prevents the wood from charring at these temperatures. As the water is removed, however, the temperature is cycled slowly down to 360 F and then subsequently below 300 F. The curing time depnds on the rate at which the water is being removed. Thus, for example, a small batch may require heating to 380 F. for only a short period, i.e. two hours, before being lowered to 300 F., because the amount of water being removed is comparatively small. However, a larger batch may require heating at temperatures ranging up to about 380 F. for about four hours before being lowered below 300 F.

Illustrations of the improved hot top compositions and the method of their preparation are shown by the following examples:

Sodium silicate solution (53.0% by weight water)--- 28 A hot top of this invention having superior body strength is prepared with the composition of Example B by completely mixing about 20 percent of the total amount of sodium silicate solution with the calcium magnesium carbonate in a batch type mixer until all the carbonate is covered with the silicate. The wood particles are added and mixed with the other ingredients until a good distribution is obtained. Following the addition of the wood, the remaining 80 percent of the silicate solution is added and the entire composition is completely mixed. This composition is plastic-like in nature and can be used to make any form or design of hot top needed to fit in the ingot mold. The mixture is shaped into the desired form, i.e. box shape with the sides sloping inward as they go upward, and then heated in an oven at a temperature of approximately 380 F. The oven has 25,000 c.f.m. of recirculating air and 3,000 c.f.m. of exhaust air. The formed composition is held in the oven at about 380 F. until about 70-90 percent of the water has been removed, and then is cycled down to approximately 360 F., and finally to a temperature below 300 F.

The body strength of the hot tops depends on the rel ative proportions of ingredients, in addition to the methods by which they are prepared. Ground wood, such as sawdust or the like, ranges between 10 to 15 percent by weight, and preferably between 12 and 14 percent by weight of the total composition. These ground wood particles usually are of a. hard wood having a particle size of 10 to 30 mesh. The smaller mesh is preferred since it gives added strength to the finished product. 'If higher percentages of wood are used, the finished product becomes soft and loses its body strength. As the amount of wood is increased beyond the critical amount, the strength of the finished product is decreased. While the reason for this is not completely understood, it is believed to be due to the increase in porosity and absorption of water which has a tendency to weaken the structure of the final product. The amount of calcium magnesium carbonate used with the wood is between 50 to 60 percent by weight, and preferably between 54 and 58 percent by weight of the total composition. The particle size of the carbonate that was found to give the highest body strength is between 10 and 100 mesh.

These small particles of calcium magnesium carbonate,

, together with the wood, generate CO gas when they come in contact with the hot metal. This gas acts as a barrier to the flow of heat from the metal and thus adds to the overall insulating eifectiveness of the hot top. Concentrated aqueous solutions of sodium silicate are used as a binder for the small particles of wood and carbonate. The sodium silicate binder ranges from 12 to 18 percent by weight, and preferably from 15 to 16 percent by weight, of the total composition. The silicate, normally, is added to the mixture as a 50 percent by weight aqueous solution.

An alternative method of preparing the hot tops of this invention comprises the treatment of the wood dust with a hydrophobic material so as to prevent the wood from absorbing water. In the normal process of preparing the composition, the wood particles usually are added to the calcium magnesium carbonate in the presence of approximately one-fifth of the silicate solution, causing the wood particles to absorb a large part of the water, thus leaving a higher concentration of silicate to react with the carbonate which causes the mixture to harden. To avoid the absorption of water, it was found that the wood can be treated with a hydrophobic material, such as a parafiin base mineral oil containing up to about percent by weight of a paraifin wax. The wax oil mixture imparts to the wood particles suflicient water resistance to avoid any substantial absorption of water and consequently avoid any premature hardening of the material.

The following is an illustration of a hot top composition containing a substantially waterproofed wood dust.

Example E I Parts by weight Calcium magnesium carbonate 56 Sodium silicate solution (50% water) 31 Hard wood dust (20 mesh) 13 The calcium magnesium carbonate together with approximately one-fifth of the total amount of silicate solution is mixed in a 'conventional mixer. The wood dust, treated by wetting with a mineral oil containing 3 percent by weight of parafiin wax, is added to the mixture. Following the addition of the hydrophobic wood dust, the remaining four-fifths of the silicate solution is added with stirring. After a complete mixing of the ingredients, the composition is formed into a box shape hot top and then placed in an oven at a temperature of approximately 360 F. After the formed product is heated in an oven for approximately two hours at 360 F., it is then cycled down to 340 F., and finally to 300 F., at which point substantially all of the water was removed. The temperature then is cycled down below 300 F. until the product is cured sufiieiently to obtain a rigid, strong product.

For purposes of illustrating the criticality of proportions, bars from the following examples were prepared and then tested to obtain their comparative body strengths:

6 Example I Percent by weight Dolomite (calcium magnesium carbonate) 56 Sodium silicate (50% aqueous solution) 31 Ground wood dust 13 Example ll Dolomite 44 Sodium silicate (50% aqueous solution) 31 Ground wood dust 25 Example 111 Dolomite 49 Sodium silicate (50% aqueous solution) 25 Ground wood dust 26 Example IV Dolomite 29 Sodium silicate (50% aqueous solution) 31 Ground wood dust 40 Example V Dolomite 20 Sodium silicate (50% aqueous solution) 30 Ground wood dust 50 Six bars-measuring 22% x 2 /4 x 1% were prepared from each of the five examples. Three of these'bars were tested for green strength and three for dried strength. All of the bars were prepared by hand in a form box by mixing the ingredients according to the method recited for Example B. The bars to be tested for dry strength were dried in an oven for approximately two hours, and then set in storage for about a week before being tested. Both the green and the dried bars were tested for their comparative body strength in the following manner.

A common destruction test comprised placing the bars across two pieces of pipe which were spaced 20 inches apart. A metal rod was placed on topof the test bars parallel to the two pieces of pipe and ten inches in from each side thereof. A container was suspended from the rod and filled with a weighting material, i.e. sand or the like, until the test bars broke. The weight of sand required to break each bar indicates the body strength of the respective composition and is given in Table I.

TABLE NO. 1

[Strengths (weight in pounds necessary to break the bar)] Three bars were dried but had so little strength that they broke during normal handling.

It is noted from the comparative data in Table 1 that the bars prepared from Example I of this invention have superior body strength. As the amount of ground wood increased from about 13 percent to about 50 percent by weight of the composition, the green and dried strengths of the composition decreased. In instances where the percent of ground wood is only doubled, for example, the

body strength is substantially decreased as shown -by comparing the strength data of Examples I, II, and III. Consequently to obtain the strength needed for purposes of this invention, it is essential that the amount of wood not exceed approximately 15 percent, and the calcium magnesium carbonate be at least 50 percent by weight of the total composition.

While the above invention has been described with reference to specific examples, it is to be understood that the invention is not intended to be limited to these examples, except as recited hereinafter in the appended claims.

What is claimed is:

1. A combustible hot top composition consisting essen tially of about 50 to 60 parts by weight of calcium magnesium carbonate, about 12 to 18 parts by weight of sodium silicate, and about 10 to 15 parts by weight of ground wood.

2. A combustible hot top composition consisting essentially of about 54 to 58 parts by weight of calcium magnesium carbonate, about 15 to 17 parts by weight of sodium silicate, and about 12 to 14 parts by weight of ground wood.

3. A combustible hot top composition of claim 2 further characterizedin that the calcium magnesium carbonate has a particle size between 10 and 100 mesh and the ground wood has a particle size between 10 and 30 mesh.

4. A combustible hot top composition consisting essentially of about 56 parts by weight of calcium magnesium carbonate, about 15.5 parts by weight of sodium silicate, and about 13 parts by weight of ground wood.

5. A combustible hot top composition of claim 4 further characterized in that the calcium magnesium carbonate has a particle size between 10 to 100 mesh and the ground wood has a particle size between 10 and 30 mesh.

6. A combustible hot top composition of claim further characterizedin that the sodium silicate is alkaline.

7. A method of preparing a hot top which comprises mixing in sequence an aqueous solution consisting essentially of approximately 2.5 to 3.5 parts by weight of sodium silicate with about 50 to 60 parts by weight of calcium magnesium carbonate, adding to this mixture to 16 parts by weight of ground wood, and finally an aqueous solution consisting essentially of approximately 10 to 14 parts by weight of sodium silicate, pouring said mixture into a mold and heating to a temperature up to about 380 F. until a majority of the water is removed and then lowering the temperature to 360 F., and finally below 300 F. after all of the water is substantially removed, to obtain a cured product.

8. A method of preparing a hot top which comprises mixing an aqueous solution consisting essentially of approximately three parts by weight of sodium silicate with about 54 to 58 parts by weight of calcium magnesium carbonate, adding to this mixture about 12 to 14 parts by weight of ground wood, and finally an aqueous solutron consisting essentially of approximately 12 to 13 parts by weight of sodium silicate; pouring said mixture into a mold and heating to a temperature of about 380 F. until a majority of the water is removed, and then lowermg the temperature to 360 F, and finally to 300 F. after all the water is substantially removed, to obtain a cured product.

9. A method of claim 8 further characterized in that the particle size of the calcium magnesium carbonate is between 10 and mesh, and the particle size of the 7 ground wood is between 10 and 30 mesh.

10. A method of preparing a hot top having different shapes and improvedbody strength which comprises mixing an, aqueous solution containing approximately three parts by weight of sodium silicate with 54 to 58 parts by weight of calcium magnesium carbonate, adding to this mixture 12 to 14 parts by weight of hard ground wood, and finally adding an aqueous solution containing approximately 12.5 parts by weight of sodium sili-'-' cate; pouring said mixture into a mold and heating at temperatures up to about 380 F. until a majority of the water is removed and then lowering the temperature to 360 F. and finally to 300 F. after substantially all of the water is removed, to obtain a cured product.

11. A process of claim 10 further characterized in that the aqueous solution of sodium silicate is alkaline.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES The Condensed Chemical Dictionary, Dolomite, fifth edition, by Arthur and Elizabeth Rose, Reinhold Pub. Co., New York, N.Y., p. 409. 

1. A COMBUSTIBLE HOT TOP COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 50 TO 60 PARTS BY WEIGHT OF CALCIUM MAGNESIUM CARBONATE, ABOUT 12 TO 18 PARTS BY WEIGHT OF SODIUM SILICATE, AND ABOUT 10 TO 15 PARTS BY WEIGHT OF GROUND WOOD.
 7. A METHOD OF PREPARING A HOT TOP WHIHC COMPRISES MIXING IN SEQUENCE AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF APPROXIMATELY 2.5 TO 3.5 PARTS BY WEIGHT OF SODIUM SILICATE WITH ABOUT 50 TO 60 PARTS BY WEIGHT OF CALCIUM MAGNESIUM CARBONATE, ADDING TO THIS MIXTURE 10 TO 16 PARTS BY WEIGHT OF GROUND WOOD, AND FINALLY AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF APPROXIMATELY 10 TO 14 PARTS BY WEIGHT OF SODIUM SILICATE, POURING SAID MIXTURE INTO A MOLD AND HEATING TO A TEMPEATURE UP TO ABOUT 380*F. UNTIL A MAJORITY OF THE WATER IS REMOVED AND THEN LOWERING THE TEMPERATURE TO 360*F., AND FINALLY BELOW 300*F. AFTER ALL OF THE WATER IS SUBSTANTIALLY REMOVED, TO OBTAIN A CURED PRODUCT. 